I use Google Nav because the software I work on is integrated with it. It does a good job integrating with search, contacts, favorites, and all of the google-map related things like shopping and event searches.
It seems to me like most of the complaints posted here are based on Google Maps, not Google Navigation, which are two separate but integrated programs. Or they're based on an older version of Google Nav. Since Google updates both probably once a month it may pay to update and try again.
Map views are customizable, Google Nav provides a "countdown" instead of an "ETA" function, and the robot voice isn't part of Nav -- it's a library that any app can use. Replace the library and all of your apps can have a different voice. Google Translate and Gmail's read-your-mail function call the same replaceable library. Turn-by-turn, automatic re-routing, and voice input are all there now, too. Those seem to be the big complaints, and they've all been addressed in updates.
Street view (yet another app) integration is really nice, too. Navigation now pops up street view when you arrive at your destination so you can see if you really are where you think you're supposed to be.
Traffic seems to be the other complaint. Traffic integration may be better with the Sprint app, I haven't had enough experience to know. It's part of Google Nav but I don't know if it's better or worse. Last time I saw it, Google did put traffic info up but didn't re-route based on it. I could see where the Sprint (actually TeleNav) app is better there.
My only beef with Google Maps/Navigation is that some of its mapping APIs are proprietary and they won't integrate with other 3rd-party mapping apps. In particular, think how cool it would be if Trapster could put its trap info on top of Nav. As it is, you have to run both mapping apps at the same time and switch back & forth. Or just listen to Trapster's audio in the background. Google maps integrates very well with other Google apps. 3rd parties not so much.
I'm not sure the argument matters since TeleNav is now included in most data plans. When it was an extra ten bucks a month, there was no contest -- Google won.
Next big trip I take, I think I'll use Google one way and TeleNav on the way home and report back.
--Qfg